Wallboard or drywall has become the dominant material in the production of interior building partitions. In particular, interior building partitions generally comprise a series of spaced vertical studs which are used as support for preformed wallboards which are attached to the studs by the screws, nails, adhesive or the like. Typically, a paper or fiberglass tape is applied to the joint between adjacent wallboard panels. In order to provide a continuous flat surface to the wall, it is necessary to “finish” the tape joint between adjacent panels. Generally such “finishing” entails the building up or accumulation of multiple layers of mastic material over the taped joint. During the finishing process, it is important that the proper crown or profile of mastic material is applied given the amount of shrinkage that occurs when each coat of mastic material dries. Crown control is dependent upon several variables including coat thickness and the amount of water mixed in the drywall compound. 
Drywall heads for applying mastic compound to finish drywall joints are known in the art. Precision Taping Tools of Arthur, Ill. produces a coater (model K-520A, K-530A, K-540A) with a flat finishing head formed with a coater body having side plates and skids or arm links adapted to engage a wall during mastic application. One wall of the coater body has a mounting plate which rotatably receives a ball member connected to a hollow arm member which allows mastic material to be moved from a pneumatic applicator to the flat finishing head. A C-spring has one end secured to a clevis integrally formed on the arm member, and another end joined to a rotatable clevis on the mounting plate. The coater body also includes a bottom cover plate, a wiper blade and a generally flat backer bar. A cam lever acts against a tensioner wire having L-shaped ends which contact the backer bar at two points. Mastic material delivered through the ball member to the interior of the coater body is controllably squeezed through a gap defined between the cover and the blade. The cam and tensioner wire are used to control the crown or profile of mastic material delivered from the finishing head.  
When using the Precision Taping Tool coater, the flat backer bar relies on the viscosity of the drywall compound, the setting of the cam tensioner wire and the deflection of the cam tensioner wire/backer bar to provide the desired mastic profile. In addition, the prior art coater uses a cam having a half-round profile with an offset pivot to provide adjustability. However, as the cam moves through its travel, it reaches a point where a small adjustment of the cam lever makes abnormally large adjustments of the backer bar/blade and vice versa. In some occasions, the prior art finishing head deflects so far as to overcrown the compound making a subsequent coat “near to impossible” without scraping or sanding the joint. Further, the prior art finishing head has its cam tension or wire contacting the backer bar at two points which sometimes allows three crowns to form instead of one. It has also been established that the prior art coater does not return to its starting position each time it is lifted off the wall. It is further noted that the prior art coater skids are subject to excessive sliding friction and wear as they are drawn over the wallboard. 
Accordingly, it is desirable to provide a drywall finishing head having an adjustment arrangement for enabling a greater predictability of crown or profile of mastic material delivered to the tape joint. It is also desirable to provide a drywall finishing head which moves along the wallboard with less friction and wear, and which returns to a start position each time it is lifted off the wallboard.  